1. Field of the Invention
This invention relates to aluminum phosphate refractory cement compositions. More particularly, this invention relates to aluminum phosphate refractory cement compositions useful for cementing tungsten-halogen lamps into glass reflectors wherein the cement composition comprises a mixture of aluminum phosphate containing excess phosphoric acid, relatively small particle size alumina and a mixture of relatively medium and coarse particle size filler.
2. Background of the Disclosure
High intensity linear types of lamps, such as the well-known tungsten halogen lamps, are being used in ever greater numbers for a variety of commercial and consumer applications. A typical lamp of this type is disclosed in U.S. Pat. No. 4,021,659 which is the pioneer patent for a particular reflector-lamp assembly wherein the lamp is a tungsten halogen lamp having a quartz envelope which is cemented into an all glass reflector. The cement must be a refractory cement, because the temperature of the lamp envelope at the seal area which is cemented into a reflector often exceeds 350.degree. C. In addition to being resistant to such high temperatures, the cement must also be electrically non-conducting, have the proper thermal conductivity and have a thermal coefficient of expansion which matches that of the reflector as closely as possible. One type of refractory cement that has been successfully used commercially for this application has been a silica-reinforced potassium silicate type of cement. Although these cements have good heat resistance with low thermal and electrical conductivity, their use with such lamp-reflector assemblies can result in corrosion problems in storage. Such cement compositions are alkaline, having a pH above 7, and, under very humid conditions and over extended periods of time, their use has resulted in corrosion of the molybdenum inlead construction in the pinch seal area of the lamp envelope which results in an inoperative lamp. Further, such cements gradually lose their strength over a period of time. Consequently, a need exists for a cement that will not adversely effect the reflector-lamp assembly.